Nuclear magnetic resonance relaxation measuremnts using the halogen ions has developed as an excellent method for measuring important properties of metal ions in metalloenzymes. This approach will be applied to making a general survey of the zinc enzymes to determine which of these provide zinc coordination positions which may interact directly with halogen ions in solution. Classification of the zinc enzymes on the basis of metal coordination will be possible from these experiments. For those in the class providing halide ion access to zinc, measurements of the zinc-halogen ion equilibrium constants will be made for the chloride, bromide and iodide ion complexes. Determination of the acidity or ionization constant for water molecules coordinated to zinc ion will be made for the pH dependence of the halogen ion line width. Rates of the metal halogen exchange reactions will be measured for the zinc halogen equilibria in the zinc enzymes and the factors which control these exchange rates will be determined. In addition to the general survey, several zinc enzymes will be investigated in greater detail. Carboxypeptidase is currently under investigation and equilirium constants for the zinc-water and zinc chloride ion equilibria have been obtained. The change in these constants on interaction of the enzyme with substrate molecules or substrate mimics is large and the extent to which substrate interactions may modulate the metal chemistry will be determined. The dependence of the metal site chemistry on alterations of the enzyme form (A,B, etc.) will be measured and the magnitude of the changes in the metal site chemistry on activation of the zymogen will be determined.